Particles aqueous dispersion and film of titanium oxide and preparation thereof

ABSTRACT

An aqueous titanium oxide-dispersed sol comprising titanium oxide particles dispersed in water, said sol comprising chloride ions in an amount of 50 to 10,000 ppm by weight as the chlorine element. Titanium tetrachloride is hydrolyzed to form an aqueous titanium oxide-dispersed sol and the chloride ion concentration thereof is controlled. Another aqueous titanium oxide-dispersed sol comprising brookite-type titanium oxide particles dispersed in water, said titanium oxide particles having an average particle size of not more than 0.5 μm and a specific surface area of not less than 20 m 2 /g. Addition of titanium tetrachloride to hot water at 75 to 100° C. followed by hydrolysis at 75° C. to the boiling point of the mixture.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an aqueous titanium oxide-dispersedsol, a titanium oxide film formed on a substrate of a ceramic, asynthetic resin or the like from said sol, specific titanium oxideparticles, and a process for preparing an aqueous titaniumoxide-dispersed sol. The titanium oxide film of the present invention istransparent and is excellent in photocatalytic activity and adhesion toa substrate.

2. Description of the Related Art

It is known that titanium dioxide (hereinafter simply referred to as“titanium oxide”) has three crystal phases, the anatase, the brookiteand the rutile type. When titanium oxide is formed by combustion oftitanium tetrachloride with oxygen in a vapor phase deposition process,anatase-type titanium oxide is formed and is stable at the lowesttemperature. When the thus formed anatase-type titanium oxide is heattreated and the temperature is raised, brookite-type titanium oxide isformed at a temperature of 816 to 1040° C. and rutile-type titaniumoxide is formed at a temperature higher than 1040° C.

Concerning a liquid process, the crystal phases of titanium oxide formedby hydrolysis of titanium tetrachloride are reported in detail byKouemon Funaki in “Kogyo Kagaku (Industrial Chemistry)” Vol. 59, No. 11,p1295. This report concluded that rutile-type titanium oxide is formedmainly from a high concentration solution and anatase-type titaniumoxide is formed from a low concentration solution. It was reported thatformation of fine brookite-type titanium oxide particles in an liquidphase process was impossible.

As seen from the above, it was difficult to stably produce brookite-typetitanium oxide in a liquid phase process. If the titanium oxide formedin a heated process is further heat treated at a high temperature,brookite-type titanium oxide may be obtained but the obtained titaniumoxide particles, have been grown by the heat treatment. Therefore, itwas difficult to obtain fine brookite-type titanium oxide crystalparticles.

As for the process for forming a titanium oxide sol, it is generallytrue that crystalline or amorphous titanium oxide particles aredispersed in a dispersing medium, or a titanium oxide precursor such asmethane alkoxide, titanium sulfate or titanium tetrachloride is mixedinto a dispersing medium, followed by neutralization or hydrolysis ofthe precursor, to form a titanium oxide sol.

A titanium oxide sol is used for producing titanium oxide particles orfor forming a titanium oxide film by coating the sol on a glass orceramic.

It is known that a titanium oxide sol is a photosemiconductor and has atransparency and an increased photocatalytic activity when its particlesize is small. The photocatalytic activity of titanium oxide hasrecently been investigated throughly. The applications of thephotocatalytic activity include removing harmful materials for cleaning,removing odor such as ammonia for deodorization, and sterilization ofmicroorganisms. Titanium oxide is used in various forms such as a bulk,particles, a film and a sol, depending on the types of the applications.When the photocatalytic activity is to be combined with thetransparency, the titanium oxide is often formed as a film. Accordingly,the titanium oxide is often used in the form of a sol for forming afilm.

It is recognized that the photocatalytic activity of titanium oxide ishigher in the rutile-type than in the anatase-type. The reason is adifference of energy gap of about 0.2 eV between the two types, as theenergy gap of the rutile-type is 3.02 eV and that of the anatase is 3.23eV (see Ceramics 31 (1996), No. 10, p817). Because of this energy gap,the anatase-type titanium oxide is preferably used as aphotosemiconductor.

As of the brookite-type titanium oxide, a pure material of brookite-typetitanium oxide has not been obtained, and it was difficult to obtainfine particles of brookite-type titanium oxide having such a highspecific surface area that they can be used as a photosemiconductorsince the particles of brookite-type titanium oxide are prepared at sucha high temperature that they are sintered.

It has been proposed that when a titanium oxide film is formed on anilluminator, for example, a glass tube of a fluorescent lump or a coverthereof, by coating it with a titanium oxide sol, organic materials suchas oil smoke, when adhered thereto, are decomposed by the photocatalyticactivity of the titanium oxide.

However, the sols produced by the processes described before rarelyprovide a titanium oxide film having a high transparency and anilluminator having a brookite-type titanium oxide film as thephotocatalyst has not been known.

When a titanium oxide film is used as the photocatalyst by forming it ona glass, plastic or other substrate, it is required that the titaniumoxide film has a high photocatalytic activity. Since the photocatalystaction is a reaction on the surface of particles, the particles shouldbe fine particles having a high specific surface area and have anexcellent crystillinity to obtain a high photocatalytic activity. It isalso required that the film is transparent when the film is applied toan illuminator. To improve the transparency, it is desired that theparticles are fine and monodispersed, as in the case of improvingphotocatalytic activity. Conventionally, the anatase-type titanium oxideis used and is made fine to solve the above problems.

It is also required that the titanium oxide film have a high adhesivityand peeling of the titanium oxide film should be prevented when it isformed on a substrate.

In the conventional process of hydrolyzing titanium tetrachloride, itwas difficult to obtain a titanium oxide sol having a very smallparticle size and an excellent crystallinity of the titanium oxideparticles in the sol and providing a high transparency when formed intoa film.

In the process of hydrolyzing titanium alkoxide, the particles of theobtained titanium oxide sol are excellent in powder characteristicsincluding very fine particle size, but the sol includes alcohol, whichinvolves a safety problem that explosion may be caused when the sol isheated to form a titanium film. To prevent the explosion, a large scaleapparatus for preventing the explosion is required and it iseconomically disadvantageous. Further, titanium alkoxide is much moreexpensive than titanium tetrachloride.

The object of the present invention is to provide a titanium oxide solwhich can provide, on a substrate, a titanium oxide film excellent inphotocatalytic activity and transparency as well as adhesion to thesubstrate, and to provide fine brookite-type titanium oxide particles.

SUMMARY OF THE INVENTION

As the result of investigation into titanium oxide films formed fromtitanium oxide sols, the present inventors have found that chloride ionscontained in a titanium oxide sol contribute to the transparency andadhesivity to the substrate of the titanium oxide film; a titanium oxidesol having a certain concentration of chloride ions provides a titaniumoxide film having improved transparency and adhesivity; and thebrookite-type titanium oxide with a large energy gap is particularlyexcellent in the photocatalytic activity.

In accordance with the present invention, the following is provided.

(1) An aqueous titanium oxide-dispersed sol comprising titanium oxideparticles dispersed in water, said sol comprising chloride ions in anamount of 50 to 10,000 ppm by weight as the chlorine element.

(2) An aqueous titanium oxide-dispersed sol comprising brookite-typetitanium oxide particles dispersed in water, said titanium oxideparticles having an average particle size of not more than 0.5 μm and aspecific surface area of not less than 20 m²/g.

(3) Brookite-type titanium oxide particles having an average particlesize of not more than 0.5 μm and a specific surface area of not lessthan 20 m²/g.

(4) A titanium oxide film which is formed on a substrate using theaqueous titanium oxide-dispersed sol as set forth in the above (1) or(2).

(5) A process for preparing an aqueous titanium oxide-dispersed sol,comprising the steps of:

forming an aqueous titanium oxide-dispersed sol by hydrolysis oftitanium tetrachloride, and

controlling an amount of chloride ions in said aqueous titaniumoxide-dispersed sol to 50 to 10,000 ppm by weight as the chlorineelement.

(6) A process for preparing an aqueous titanium oxide-dispersed sol,comprising the steps of:

adding titanium tetrachloride to hot water at a temperature of 75 to100° C., and

hydrolyzing the titanium tetrachloride at a temperature in a range of75° C. to a boiling point of the solution or sol, to form an aqueous solof brookite-type titanium oxide particles.

(7) A process for preparing brookite-type titanium oxide particles, inwhich the aqueous sol of brookite-type titanium oxide particles of theabove (6) is filtered and dried.

BRIEF DESCRIPTION OF THE DRAWING

The FIGURE shows a-reactor equipped with a reflux cooler, which is usedfor producing a titanium oxide sol in an example of the presentinvention.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

A first aqueous titanium oxide-dispersed sol of the present invention isa sol which provides a titanium oxide film having not only excellentphotocatalytic activity but also increased adhesion to a substrate andtransparency, and is characterized by containing chloride ions in anamount of 50 to 10,000 ppm, preferably 100 to 4,000 ppm as the chlorineelement.

In the process of hydrolyzing titanium chloride to obtain an aqueoustitanium oxide-dispersed sol, hydrogen chloride is formed by thereaction. The hydrogen chloride is almost dissociated to chloride ionand hydrogen ion in the sol. In general, the hydrogen chloride escapesfrom the reaction system during the hydrolysis under heating. Further,when hydrogen chloride in the sol increases to a certain level in thehydrolysis, dechlorination treatment is usually carried out on the solto remove hydrogen chloride, since if the sol contains hydrogenchloride, various problems occur in obtaining titanium oxide particlesor a titanium oxide film from the sol. The relationship between thechloride ions in the sol and the characteristics of the titanium oxidefilm have not been considered in the prior art and there is notechnology which controls the chloride ions in the sol from thisviewpoint.

If the chloride ions contained in the aqueous titanium oxide-dispersedsol are in an amount of less than 50 ppm as the chlorine element, thetitanium oxide film formed on a substrate from the sol has poor adhesionto the substrate. Particularly when the titanium oxide film is heattreated, a difference in the adhesion of the film to the substrate,depending on whether or not the chloride ions are contained in an amountof not less than 50 ppm, appears. In the present invention, the adhesionof the film to a substrate is represented by the peeling force of thefilm from the substrate and the hardness of the film. On the other hand,if the amount of the chloride ions in the sol increases to more than10,000 ppm as the chlorine element, the transparency of the film isreduced. A preferable range is 100 to 4,000 ppm.

The action of the chloride ions is not clear, but it is supposed that asthe electrical repulsion between titanium oxide particles increases inthe titanium oxide sol, the dispersibility of the particles is improvedso that the transparency and peeling strength of the film are improved.

As the titanium oxide particles in the aqueous titanium oxide-dispersedsol are finer, the photocatalytic activity and the transparency of thetitanium oxide film are improved. It is preferable from thephotocatalytic activity that the titanium oxide particles arecrystalline. However, if the particle size of the titanium oxideparticles is too small, such particles are difficult to produce.Accordingly, an average particle size of titanium oxide particles in asol is preferably in a range of 0.01 to 0.1 μm.

A second aqueous titanium oxide-dispersed sol of the present inventionis a sol which provides a titanium oxide film having improvedphotocatalytic activity and transparency and is characterized in thatthe titanium oxide particles dispersed in water are brookite-typetitanium oxide particles having an average particle size of not morethan 0.5 μm, preferably 0.01 to 0.1 μm and a specific surface area ofnot less than 20 m²/g. The brookite-type titanium oxide particles havean energy gap of 3.23 eV or more.

As to the particle size of the titanium oxide particles, it ispreferable for the transparency that the titanium oxide particles aremonodispersant with an average particle size of not more than 0.5 μm,preferably 0.01 to 0.1 μm. Even if the specific surface area of theparticles is large, agglomerates of primary particles do not provide atransparent film of titanium oxide.

In the prior art, the brookite-type titanium oxide cannot be producedexcept by a process in which anatase-type titanium oxide is heattreated. If the brookite-type titanium oxide is produced by the heattreatment, the brookite-type titanium oxide particles are grown to alarge particle size by the heat treatment, which therefore has not beenused to form a titanium oxide film.

In a sol in which the above brookite-type titanium oxide particles aredispersed in water, the chloride ions may be contained in an amount of50 to 10,000 ppm as the chlorine element, by which the titanium oxidefilm formed from the sol can be excellent not only in photocatalyticactivity but also in adhesion to a substrate.

In the above first and second aqueous titanium oxide-dispersed sols ofthe present invention, if the concentration of the titanium oxideparticles is too high, the particles agglomerate and the sol becomesunstable. If the concentration of the titanium oxide particles is toolow, there are often problems. For example, the step of forming atitanium oxide film by coating takes a long time. Therefore, theconcentration of the titanium oxide particles is appropriate in a rangeof 0.05 to 10 mol/l.

By filtering, washing and drying the aqueous titanium oxide-dispersedsol of the present invention, titanium oxide particles can be obtained.The brookite-type titanium oxide particles thus obtained have an averageparticle size of not more than 0.5. μm, preferably 0.01 to 0.1 μm and aspecific surface area of not less than 20 m²/g. They have an energy gapof 3.23 eV or more.

When the aqueous titanium oxide-dispersed sol is used to form a titaniumoxide film, it is preferred that a water-soluble polymer is added in asmall amount, for example, about 10 to 10,000 ppm, to improve thefilm-forming capability or coatability thereof. Preferable water-solublepolymers include polyvinylalcohol, methylcellulose, ethylecellulose,CMC, starch, etc.

The aqueous titanium oxide-dispersed sol of the present invention can becoated on a substrate of various materials to form a titanium oxide filmon the surface of the substrate. The substrate is not limited and may beceramic, metal, plastic, wood, paper, etc.

The substrate may be a catalyst carrier of alumina, zirconia, etc. onwhich the titanium oxide film be provided as a catalyst, by which acatalyst is produced. Also, the substrate may be a glass tube or aplastic cover of an illuminator such as a fluorescent lamp, on which thetitanium oxide film can be formed. This titanium oxide film istransparent and has a photocatalytic activity, so that the film candecompose organic materials such as oil smoke without shielding thelight, which is therefore useful to prevent dirt of a glass tube or aplastic cover. If such a titanium oxide film is formed on a window paneor wall of a building, dirt on the pane or wall can be also prevented.If the film is provided on a window pane or wall of a tall building, thenecessity of cleaning can be removed or reduced so that it is useful toreduce cost for maintenance of the building.

The methods for applying an aqueous titanium oxide-dispersed sol to asubstrate include immersion of a substrate in a sol, spraying a sol ontoa substrate, brush coating a sol on a substrate, and so on. Thethickness of the applied sol is appropriately 0.01 to 0.2 mm. After theapplication of the sol to a substrate, the water content of the sol isremoved by drying to obtain a titanium oxide film. This film may be usedas a catalyst, etc., as mentioned above.

When the substrate is of a heat resistant material, for example, glass,the titanium oxide film formed on the substrate may be heat treated. Bythis heat treatment, the film may be adhered to the substrate morestrongly and have a higher hardness. The temperature of heat treatmentis preferably not less than 200° C. The upper limit of the heattreatment is not particularly set and can be determined based on theheat resistance of the substrate. However, the hardness and the adhesiveforce to the substrate of the film do not increase even if thetemperature is very high. Therefore, a temperature up to about 800° C.is appropriate. In the case of brookite-type titanium oxide, atemperature of not higher than 700° C. is appropriate to maintain thecrystal phase of brookite-type titanium oxide.

Alternatively, the adhesive force of the transparent titanium oxide filmto the substrate can be increased without heat treatment, by adding anappropriate adhesive to an aqueous titanium oxide-dispersed Sol of thepresent invention. An appropriate adhesive includes an organicsilica-containing compound such as alkylsilicate. The amount of theadhesive may be in an amount (as SiO₂) of 1 to 50% by weight of thetitanium oxide of the titanium oxide sol. If the amount of the adhesiveis less than 1% by weight, the desired effect cannot be obtained. If theamount of the adhesive is more than 50% by weight, a very high adhesioncan be obtained but the photocatalytic activity of the film is lost asthe titanium oxide particles are covered by the adhesive, which is notpreferable. The adhesive may be added to the sol just prior to use(application or coating) or may be previously added when the sol isprepared, considering the nature of the adhesive.

The atmosphere of the heat treatment is not particularly limited and maybe air. The time for the heat treatment is not particularly limited andmay be, for example, 1 to 60 minutes. The titanium oxide film obtainedafter the heat treatment is about 0.05 to 1.0 μm in thickness when thesol is applied in an amount as mentioned above.

The preparation of the aqueous titanium oxide-dispersed sol of thepresent invention is described below.

The process for preparing the first aqueous titanium oxide-dispersed solof the present invention is not particularly limited as long as theprepared sol contains chloride ions in the above-mentioned amount. Forexample, titanium alkoxide can be hydrolyzed to form an aqueous titaniumoxide-dispersed sol containing a small amount of alcohol, to which HClor the like be added to control the chloride ion to the above-mentionedconcentration. However, it is preferred that titanium tetrachloridewhich forms hydrogen chloride by hydrolysis is used.

The second aqueous titanium oxide-dispersed sol of the present inventionis obtained by hydrolyzing titanium tetrachloride under certainconditions.

It is preferred that hydrogen chloride produced by the above hydrolysisbe prevented from escaping from the reactor and be maintained in thesol. When titanium tetrachloride is hydrolyzed while the producedhydrogen chloride leaks out, it is difficult to make the particle sizeof titanium oxide particles in the sol small and the crystallinity ofthe titanium oxide particles obtained is poor.

It is not necessary to completely prevent hydrogen chloride produced bythe hydrolysis escaping or leaking from the reactor and suppression ofthe escape or leakage is sufficient. The method of prevention orsuppression is not limited. For example, evacuation or pressurereduction may be adopted, but the easiest and most effective method ishydrolysis in a reactor equipped with a reflux cooler. The FIGURE showssuch a reactor. In the FIGURE an aqueous solution of titaniumtetrachloride 2 is charged in a reactor 1 which is equipped with areflux cooler 3. The reactor 1 is also equipped with a stirrer 4, athermometer 5 and a heater 6. As hydrolysis produces vapor of hydrogenchloride and water, most of the vapor is condensed by the reflux coolerand returned to the reactor, so that the hydrogen chloride hardlyescapes from the reactor.

If the concentration of the titanium tetrachloride in the aqueoustitanium tetrachloride solution to be hydrolyzed is too low, theproductivity is low and the efficiency of forming a titanium oxide filmfrom the obtained aqueous titanium oxide-dispersed sol is low. If theconcentration of the titanium tetrachloride in the aqueous titaniumtetrachloride solution to be hydrolyzed is too high, the reactionbecomes vigorous so that it is difficult to make the particle size ofthe titanium oxide particles small and the dispersability is lowered,which is not suitable for a material for forming a transparent film.Accordingly, a method of forming a sol having a high titanium oxideconcentration by hydrolysis, followed by diluting with a large amount ofwater to control the concentration of titanium oxide to 0.05 to 10mol/l, is not preferred. It is desired that the concentration oftitanium oxide is controlled to this range when the sol is formed. Toattain this, the concentration of titanium tetrachloride in the aqueoustitanium tetrachloride solution to be hydrolyzed is controlled to bealmost equal to the concentration of the titanium oxide to be formed,i.e., approximately 0.05 to 10 mol/l, and if necessary, addition of asmall amount of water or condensation in the following step to controlthe concentration of the titanium oxide to 0.05 to 10 mol/l is done.

The temperature of hydrolysis is preferably in a range of not lower than50° C. to the boiling point of the aqueous titanium tetrachloridesolution. At a temperature of lower than 50° C., a long time isnecessary for the hydrolysis. After the temperature is raised to theabove temperature, hydrolysis is carried out at the temperature forabout 10 minutes to 12 hours. The time for maintaining a certaintemperature for hydrolysis may be shorter as the temperature ofhydrolysis is lower.

The hydrolysis may be conducted by heating a mixture of water andtitanium tetrachloride in a reactor to the predetermined temperature, oralternatively, by previously heating water in a reactor and addingtitanium tetrachloride to the heated water to raise it to thepredetermined temperature.

By the above hydrolysis titanium oxide of brookite-type or a mixture ofbrookite-type with anatase-type and/or rutile-type is generallyobtained. To increase the content of brookite-type titanium oxide, it isappropriate that water is previously heated to 75 to 100° C., titaniumtetrachloride is added to this water and hydrolysis is carried out at atemperature of from 75° C. to the boiling point of the solution. Inaccordance with this process, the content of brookite-type titaniumoxide in the produced total titanium oxide can be increased to not lessthan 70% by weight.

The rate of raising the temperature is preferably not less than 0.2°C./min, more preferably not less than 0.5° C./min, since the producedtitanium oxide particles become finer as the rate of raising thetemperature increases.

The preparation of the aqueous titanium oxide-dispersed sol of thepresent invention may be conducted in a batch system or a continuoussystem in which titanium tetrachloride and water are continuously addedto a continuous-type reactor, from the opposite end of which thereaction solution is removed and then sent to the dechlorinationtreatment.

In the first aqueous titanium oxide-dispersed sol of the presentinvention, the obtained aqueous titanium oxide-dispersed sol is then,depending on necessity, subjected to dechlorination treatment or, ifacceptable, water is added or removed to control the chloride ionconcentration to 50 to 10,000 ppm.

In the second aqueous titanium oxide-dispersed sol of the presentinvention, the obtained aqueous titanium oxide-dispersed sol may be thensubjected to dechlorination treatment or, if acceptable, water is addedor removed to control the chloride ion concentration to 50 to 10,000ppm, if desired or necessary.

The dechlorination treatment maybe a known process such aselectrodialysis, treatment with an ion exchange resin, or electrolysis.The level of the dechlorination treatment can be detected by pH. Whenthe chloride ion concentration is 50 to 10,000 ppm, the pH of the sol isabout 5 to 0.5 and when the chloride ion concentration is in a preferredrange of 100 to 4,000 ppm, the pH of the sol is about 4 to 1.

An organic solvent may be added to the aqueous titanium oxide-dispersedsol of the present invention to disperse the titanium oxide particles ina mixture of water and an organic solvent.

When a titanium oxide film is formed from the aqueous titaniumoxide-dispersed sol of the present invention, it is preferred that theaqueous titanium oxide-dispersed sol is directly used to form a titaniumoxide film. A process of first forming titanium oxide particles from theaqueous titanium oxide-dispersed sol, followed by dispersing theobtained titanium oxide particles in water to form a titanium oxide sol,and then using the thus obtained sol to form a titanium oxide film, isnot preferred. This is because titanium oxide particles have a highersurface activity as the particles are finer, but the finer titaniumoxide particles are difficult to disperse in water. That is, they becomeagglomerates which provides a titanium oxide film having a loweredtransparency or photocatalytic activity.

EXAMPLES

The present invention is now described with reference to examples of thepresent invention, to which the present invention is, of course, notlimited.

Examples 1 to 6

Water was added to titanium tetrachloride (purity: 99.9%) to control theconcentration of titanium tetrachloride of the solution to 0.25 mol/l(reduced to titanium oxide: 2% by weight), while the aqueous solutionwas cooled by a cooler such as ice to prevent the temperature of thesolution from exceeding 50° C. One liter of the aqueous solution wascharged in a reactor equipped with a reflux cooler as shown in theFIGURE and heated to the boiling point (104° C.) of the solution andhydrolysis was conducted for 60 minutes by maintaining that temperature.The obtained sol was cooled and then subjected to electrodialysis toremove chloride produced in and remaining after the reaction to thechloride ion concentrations as shown in Table 1. The electrodialysis wascarried out using an electrodialysis device G3, manufactured by AsahiKasei Kogyo K.K., while the pH of the sol was monitored.

Observation of the particles in the sols demonstrated that the averageparticle sizes of the particles were from 0.015 to 0.018 μm.

X-ray diffraction of the particles revealed that the particles werecrystalline titanium oxide.

Comparative Examples 1 and 2

The procedures of Examples 1 to 6 were repeated but the chloride ionconcentrations were controlled to 30 ppm (Comparative Example 1) and15,000 ppm (Comparative Example 2).

To the thus produced aqueous titanium oxide-dispersed sols with thecontrolled chloride ion concentrations of Examples 1 to 6 andComparative Examples 1 and 2, a water-soluble polymer ofpolyvinylalcohol as a film-forming agent was added in an amount of 1,000ppm based on the weight of the sols. These sols with the chloride ionconcentrations of 50 to 10,000 ppm were stable and did not showprecipitation of titanium oxide particles even after one day (Examples 1to 6). However, the sol with the chloride ion concentrations of 30 ppmshowed agglomeration of the titanium oxide particles in the sol and thesol with the chloride ion concentrations of 15,000 ppm resulted in atitanium oxide film with a light white color.

(Comparative Examples 1 and 2).

Using the sols of Examples 1 to 6 and Comparative Examples 1 and 2,titanium oxide films were formed on glass plates by dip coating a sol ona glass plate followed by drying and heat treating at 500° C. for 1hour. The obtained titanium oxide films were 0.15 μm.

Rietveld analysis of the powder X ray diffraction patterns of thetitanium oxide revealed that the titanium oxide before the heattreatment was a mixture of about 50% by weight of anatase-type titaniumoxide and about 50% by weight of brookite-type titanium oxide, and thetitanium oxide after the heat treatment at 800° C. or more was a singletype of the rutile-type titanium oxide.

(Evaluation of film)

The light permeability, photocatalytic activity, and adhesive force to aquartz glass plate of a titanium oxide film obtained from each of theaqueous titanium oxide-dispersed sols of Examples and ComparativeExamples, were evaluated.

The light permeability was measured for a titanium oxide film formed ona quartz glass plate by using a spectrophotometer, manufactured by NihonBunkoh (Japan Spectroscopy) K.K., while continuously changing thewavelength from 700 nm to 200 nm. The light permeability of the film at550 nm was used as the light permeability of the film in the presentinvention. The results are shown in Table 1.

The photocatalytic activity of the titanium oxide was measured by makinga reactor using a quartz glass plate coated with a titanium oxide film,charging 5 mol/l of oxalic acid in the reactor, irradiating the oxalicacid with a mercury lamp while blowing oxygen into the oxalic acid, anddetermining the amount of the decomposed oxalic acid by redox titratingpotassium permanganate. The results are shown in Table 1.

The adhesivity of the film to a substrate was measured by the pencilhardness method and by the XY-matrix cut film-peeling method (JISK5400). The peeling strength in the XY-matrix cut film-peeling method isrepresented by the rate of the non-peeled sections to the total cutsections.

TABLE 1 Sample No. Chloride ion concentration in titanium oxide sol [ppm(pH)] Light permeability (%) Decomposition of oxalic acid (%) Pencilhardness $\begin{matrix}{{P{eeling}}\quad {strength}} \\\left\lbrack \frac{{{non}–{peeled}}\quad {sections}}{{total}\quad {of}{\quad \quad}100\quad {sections}} \right\rbrack\end{matrix}$

EX. 1 50 (5) 96 43  H 95 Ex. 2 100 (4) 96 44 2H 100 Ex. 3 1,000 (1.7) 9644 4H 100 Ex. 4 4,000 (1.0) 95 45 6H 100 Ex. 5 7,000 (0.8) 94 45 6H 100Ex. 6 10,000 (0.7) 90 45 6H 100 Com. Ex. 1 30 (5.5) 96 43  B 70 Com. EX.2 15,000 (0.5) 55 42 6H 95

Example 7 and 8 and Comparative Examples 3 and 4

The procedures as in Example 1 and Comparative Example 1 were repeated,except that the titanium oxide film was formed on a plastic substrate ofpolyethyleneterephtalate and the heat treatment was not conducted andthe film was dried at 100° C. The evaluations were carried out in thesame manner as in Examples and Comparative Examples.

The results are shown in Table 2.

TABLE 2 Sample No. Chloride ion concentration in titanium oxide sol [ppm(pH)] Light permeability (%) Decomposition of oxalic acid (%) Pencilhardness $\begin{matrix}{{P{eeling}}\quad {strength}} \\\left\lbrack \frac{{{non}–{peeled}}\quad {sections}}{{total}\quad {of}{\quad \quad}100\quad {sections}} \right\rbrack\end{matrix}$

Ex. 7 100 (4) 95 44 HB 90 Ex. 8 4,000 (1.0) 95 43 3H 95 Com. Ex. 3 30(5.5) 93 43 HB 70 Com. Ex. 4 15,000 (0.5) 55 42 3H 83

Example 9

954 ml of distilled water was charged in a reactor equipped with areflex cooler as shown in the FIGURE and heated to 95° C. While thestirring rate was kept to about 200 rpm, 46 ml of an aqueous solution oftitanium tetrachloride(titanium element content: 16.3% by weight;density: 1.59; purity: 99.9%) was added dropwise into the reactor at arate of about 5 ml/min. The temperature of the reaction solution waskept constant during the reaction. As a result, the concentration oftitanium tetrachloride was changed to 0.25 mol/l (reduced titanium oxideconcentration: 2% by weight).

In the reactor, the reaction solution became clouded soon after startingthe addition of titanium tetrachloride, but the temperature of thereaction solution was kept constant and, after finishing the addition,raised to near the boiling point (104° C.) and kept at this temperaturefor 60 minutes to complete and finish the reaction. After cooling,produced and remaining chlorine was removed by electrodialysis to a pHof 2 (chloride ion: 600 ppm), followed by adding a water soluble polymerof polyvinylalcohol in an amount of 0.1% based on the weight of titaniumoxide, to obtain a titanium oxide sol.

The sol was filtered and vacuum dried at 60° C. to obtain a powder,which was analyzed by X ray diffraction to reveal that the titaniumoxide contained 96.7% by weight of brookite-type, 0.9% by weight ofrutile-type, and 2.4% by weight of anatase-type titanium oxides.

Observation of the powder by a transmission type electromicroscoperevealed that the average particle size of the primary particle was 15nm.

The specific surface area of the powder was 100 m²/g by the BET method.

The above sol was spin-coated over a quartz glass plate and dried in adrier at 100° C. to obtain a transparent film. The light permeability ofthe quartz glass plate with the film was over 95% in the range of thevisible light, which demonstrated that the plate with the film wascompletely transparent. The plate with the film demonstrated anabsorption in an ultraviolet region, which revealed by the absorptionend of light that the energy gap was 3.75 eV. The relationship betweenthe energy gap and the absorption end of light is shown by the followingformula (1):

λ=1239/ Eg  (1)

where λ stands for the absorption end of light in the unit of nm, and Egstands for the energy gap in the unit of eV.

Example 10

The procedures of Example 9 were repeated but the reaction temperatureduring the addition of titanium tetrachloride was 75° C.

The obtained titanium oxide powder was analyzed by the X ray diffractionto reveal that it contained 75% by weight of brookite-type and 25% byweight of rutile-type titanium oxides.

Observation of-the powder by a transmission type electromicroscoperevealed that the average particle size of the primary particle was 10nm.

The specific surface area of the powder was 120 m²/g by the BET method.

The above sol having a pH of 1 (chloride ion: 3000 ppm), obtained by theelectrodalysis, was coated over a quartz glass plate and heat treated at500° C. to obtain a transparent film. The transparent film was also amixture of brookite-type and rutile-type titanium oxides, as determinedby the thin film X ray diffraction method. The light permeability of thequartz glass plate with the film was over 95% in the range of thevisible light, which demonstrated that the plate with the film wascompletely transparent. The plate with the film demonstrated anabsorption in a ultraviolet region, which revealed by the absorption endof light that the energy gap was 3.30 eV.

Comparative Example 5

Anatase-type titanium oxide particles having a primary particle size of7 nm were dispersed in water with an ultrasonic wave dispersing deviceto an aqueous solution of titanium oxide having a concentration oftitanium oxide of 2% by weight as in Example 9, during whichhydrochloric acid as a coagulant was added to control the pH to 1,followed by carrying out the same procedures as in Example 9 to obtainan aqueous titanium oxide-dispersed sol. The sol was coated on a glassplate and dried at 100° C. to form a transparent film.

Comparative Example 6

An aqueous titanium oxide-dispersed sol was prepared in the sameprocedures as in Comparative Example 5, except that rutile-type titaniumoxide particles having a primary particle size of 50 nm were used. Thesol exhibited precipitation of titanium oxide particles as inComparative Example 5 and the particles were redispersed usinghydrochloric acid as a coagulant. Since a titanium oxide film formed bythe supernatant after the precipitation did not show a photocatalyticactivity, the sol, soon after it was prepared, was subjected todispersing treatment with an ultrasonic dispersing device, and theobtained sol was used to form a titanium oxide film on a glass plate inthe same manner as in Example 9 and the photocatalytic activity of thefilm was evaluated.

(Evaluation of film)

The photocatalytic activities of the titanium oxide films obtained fromthe sols of Examples 9 to 11 and Comparative Example 5 and 6 wereevaluated by the oxalic acid decomposition method. The results are shownin Table 3.

TABLE 3 Rate of decomposition (%) (after 4 hours irradiation) Example 955 Example 10 48 Example 11 50 Comparative Example 5 30 ComparativeExample 6 not determined since the substrate was clouded

In Comparative Example 5, the surface of the glass substrate was notuniform as agglomerates of titanium oxide were formed.

In Comparative Example 6, the photocatalytic activities of the titaniumoxide film was not determined since a transparent titanium oxide filmwas not obtained.

Example 12

In the same manner as in Example 9, 0.25 mol/l (reduced to titaniumoxide: 2% by weight) of an aqueous titanium tetrachloride solution wassubjected to hydrolysis. The resultant reaction solution was condensedto a titanium oxide concentration of 10% by weight, electrodialysis wascarried out to remove the remaining chlorine to a pH of 2 (chloride ionconcentration of about 600 ppm), and tetramethylorthosilicate Si(OCH₃)₄as an adhesive was added to the sol in an amount as SiO₂ of 5% byweight, to obtain a titanium oxide sol.

Example 13

The procedures up to the condensation and electrodialysis in theprocedures in Example 12 were repeated, followed by diluting withisopropylalcohol to 5 times and adding tetraethylorthosilicateSi(OC₂H₅)₄ as an adhesive in an amount, as SiO₂, of 20% by weight, toobtain a titanium oxide sol.

Example 14

The procedures of Example 12 were repeated but tetrapropylorthosilicateSi(OC₃H₂)₄ was substituted for tetramethylorthosilicate and added in anamount, as SiO₂, of 35% by weight, to obtain a titanium oxide sol.

Comparative Example 7

The procedures of Example 14 were repeated but tetrapropylorthoslicatewas added in an amount, as SiO₂, of 55% by weight, to obtain a titaniumoxide sol.

(evaluation of film)

Each of the sols obtained in Examples 12 to 14 and Comparative Example 7was spin-coated over a quartz glass plate and allowed to stand fordrying to obtain a transparent film. The light permeabilities of thequartz glass plates with the film were over 95% in the range of thevisible light, which demonstrated that the plates with the films werecompletely transparent.

The pencil hardness test and the adhesion test were made on the quartzglass plates with the titanium oxide films. The results are shown inTable 4.

TABLE 4 Sample No. Silicon oxide/ titanium oxide ratio (% by weight)Decomposition of oxalic acid (%) Pencil hardness (%) $\begin{matrix}{{P{eeling}}\quad {strength}} \\\left\lbrack \frac{{{non}–{peeled}}\quad {sections}}{{total}\quad {of}{\quad \quad}100\quad {sections}} \right\rbrack\end{matrix}$

Ex. 12 5 50 5H 100 Ex. 13 20 50 5H 100 Ex. 14 35 40 6H 100 Com. Ex. 7 550 7H 100

What is claimed is:
 1. An aqueous titanium oxide-dispersed solcomprising chloride ions and titanium oxide particles dispersed inwater, wherein said sol comprises said chloride ions in an amount of 50to 10,000 ppm by weight as the chlorine element; said sol comprises saidtitanium oxide particles in an amount of 0.05 to 10 mol/l as thetitanium oxide; said titanium oxide particles have an average particlesize of not more than 0.5 μm and a specific surface area of not lessthan 20 m²/g; and said titanium oxide particles, as analyzed by x-raydiffraction after being filtered from the sol and vacuum dried, comprisetitanium oxide particles having a brookite structure in an amount of 70%by weight or more based upon the total weight of the titanium oxideparticles.
 2. The aqueous titanium oxide-dispersed sol according toclaim 1, wherein said titanium oxide particles are crystalline and havean average particle size of 0.01 to 0.1 μm.
 3. The aqueous titaniumoxide-dispersed sol according to claim 1, wherein said sol contains awater-soluble polymer in an amount of 10 to 10,000 ppm by weight.
 4. Theaqueous titanium oxide-dispersed sol according to claim 1, wherein saidbrookite titanium oxide has an energy gap of not less than 3.23 eV. 5.The aqueous titanium oxide-dispersed sol according to claim 1, whereinsaid sol further contains alkyl silicate as an adhesive in an amount assilicon oxide of 1 to 50% by weight based on the weight of titaniumoxide.